1. Field of the Invention
This invention relates to a method and an apparatus for producing a stepped tube which is formed with a stepped wall portion of a larger diameter integrally around its circumference at a desired position in the longitudinal direction of the tube.
2. Description of the Prior Art
Stepped tubes are used mainly in the nuclear industry, but it has been extremely difficult to manufacture the tubes in compliance with the very strict standards of accuracy which are generally required in that industry.
More specifically, it has been found very difficult to form a stepped wall portion of required dimensions at a predetermined position in a tube without causing torsional or bending deformations thereto. Straightening of the tube subsequent to a forming operation is inevitable, especially in a case of a polygonal stepped tube, for example, a tube of a hexagonal shape in section. Therefore, the accuracy of the forming operation governs the accuracy of the ultimate product, so that the shape and dimensions after a forming operation should be within the ranges of prescribed standards.
Besides the accuracy in shape, the tube is required to comply with the accuracy required regarding the uniformity of physical properties, for example, to have uniform distribution of hardness in the straight and stepped portions of the tube. In order to ensure unformity of hardness, it is necessary to work the straight and stepped portions of the tube at the same working rate, which, however, involves extraordinary technical difficulties.